Brilliant, this is exactly what I needed to know, thanks!

use one of the online 'LED calculator's, google that, do the calculation, and you should come up with the precise value for your particular LED.  adjust upwards from there, just don't go below the value it gives you.

assume the voltage to be 5V coming off the arduino, 5.5V if you want to be a little safer.  and, calculate it for the particular LEDs you bought, you should be able to look up the datasheet for those

its fine to do.  you'll increase the overall current draw of the device, but very slightly.  resistors reduce the flow of current, which also has the effect of lowering voltage.  so lessening the resistance will adjust both those values as well (this is Ohm's law, fyi). 

so, if you are using a console to power the arduinoboy (rather than USB or its own power supply), this is disadvantageous for you to do.  the current draw difference is pretty minimal, but still, there is *some* difference.

I've been following the one on Trash80's google code page.

What schematic are you going off of?!

Hey again everyone!

I got a quick question, I'm in the midst of building an arduinoboy and (This may sound stupid) but do I need the 2Kohm resistors before each LED? 2k sounds a little high and I'd want the brightness to be as high as possible, but I don't know if it is safe to do this (Multimeter is out of action so I can't test the pd coming out of the pins ^^'). has anyone foregone the resistors before and came out unscathed?

Many thanks in advance!